Field of Invention
The present invention relates, in general, to cement manufacturing facilities that utilize baghouses (dust collectors) to con-currently control particulate matter and certain other emissions. Other processes in which this invention can apply include lime manufacturing, pulp and paper, electric power generation, and other similar industries.
Description of the Related Art
In the cement manufacturing industry, the recently promulgated Portland Cement National Emissions Standards for Hazardous Air Pollutants (PC NESHAP) has forced most plants into significant capital investments to comply with new and more strict emissions standards relating to Mercury (Hg), Hydrochloric Acid (HCl), Total Hydrocarbons (THC), Particulate Matter (PM) and Dioxin-Furans (D/F). In the case of mercury, the PC NESHAP standard is 55 lbs Hg/million tons clinker for existing plants and 21 lbs Hg/million ton clinker for new plants, measured on a 30-day rolling average. Clinker is the intermediate product after the pyro-processing step in the cement manufacturing process.
One general problem that cement manufacturing companies and plants face is achieving compliance with these rules, regulations, permits, etc. in a cost-effective manner. Conventional treatment options available for plants that currently cannot achieve the above Hg standard include: 1) switching of raw materials or fuel to a lower mercury material, 2) dust shuttling which entails removal or purge of material from a point in the process in which mercury is concentrated (such as the main kiln baghouse), or 3) sorbent injection (typically Activated Carbon) and material removal. Similar situations exist when controlling other pollutants such as Hydrochloric Acid (HCl) and Sulfur Dioxide (SO2).
Removal of pollutants such as mercury using activated carbon injection (“ACI”) is typically accomplished by methods including: 1) capturing mercury from process gas streams typically in a main kiln baghouse using activated carbon injection. Sorbents such as activated carbon are injected into the main kiln baghouse, mercury attaches to the activated carbon, then absorbed/adsorbed mercury is removed along with certain amounts of kiln raw meal material. This removed material is typically disposed or recycled into other parts of the process, such as a material into the finish milling process or cement. Another method includes 2) injection of activated carbon injected into a ‘Polishing Baghouse’. A Polishing Baghouse is a separate baghouse installed in series (after) the main kiln baghouse. The Polishing Baghouse approach provides a greater potential for mercury removal as it provides a means to perform ACI during all phases of operation (mill-on or compound operation and during mill-off or direct operation). For in-line raw mill systems, it is not possible/practical to waste large amounts of material captured in the main baghouse during “raw mill on” operation, as that material is recycled/used as feed for the pyro-processing of materials into a cement clinker. The Polishing Baghouse also allows for high capture rates of mercury per unit of activated carbon because the unit receives a ‘clean’ airstream (free of particulate matter). This ‘clean’ airstream allows reduced frequency of bag cleaning cycles, which means the carbon can stay in place for longer periods allowing more mercury capture per unit of carbon, when compared with the first capturing method described above. Additionally, the available surface area for mercury capture is generally much higher with a Polishing Baghouse, as a thicker filter cake of carbon is more economically maintained on the filter bags.
However, Polishing Baghouses involve high capital investment and operating costs, because it basically duplicates the very expensive main baghouse system.
Accordingly, as recognized by the present inventors, new systems and processes are needed to utilize the benefits of polishing baghouses while reducing the capital costs and expenses associated with conventional polishing baghouses.